An organic electroluminescent device (hereinafter referred to as an organic EL device) in its simplest structure is generally constituted of a light-emitting layer sandwiched between a pair of counter electrodes and functions by utilizing the following phenomenon. Upon application of an electrical field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and they recombine in the light-emitting layer with emission of light.
In recent years, organic thin films have been used in the development of organic EL devices. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been focused on commercial applications to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device containing a hole-transporting layer of an aromatic diamine and a light-emitting layer of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state is expected to enhance the luminous efficiency approximately three to four times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer has been investigated, but these compounds merely produced luminance at an extremely low level. Thereafter, europium complexes were tried to utilize the excited triplet state, but failed to emit light at high efficiency. In recent years, as is mentioned in the patent document 1, a large number of researches are conducted with the objective of enhancing the luminous efficiency and extending the service life while mainly utilizing organic metal complexes such as iridium complexes.    Patent document 1: JP2003-515897 A    Patent document 2: JP2001-313178 A    Patent document 3: JP3711157 B    Patent document 4: JP2006-510732 A    Non-patent document 1: Applied Physics Letters, 2003, 83, 569-571    Non-patent document 2: Applied Physics Letters, 2003, 82, 2422-2424
A host material to be used together with the aforementioned dopant material becomes important in order to enhance the luminous efficiency. Of the host materials proposed thus far, a typical example is 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP) which is a carbazole compound presented in the patent document 2. CBP exhibits relatively good luminous characteristics when used as a host material for green phosphorescent emitters, typically tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3). On the other hand, CBP fails to perform at sufficiently high luminous efficiency when used as a host material for blue phosphorescent emitters. This is because the energy level of the lowest triplet excited state of CBP is lower than that of common blue phosphorescent emitters and the triplet excitation energy of a blue phosphorescent emitter in use is transferred to CBP. That is to say, if a phosphorescent host material were designed to have triplet excitation energy higher than that of a phosphorescent emitter, the triplet excitation energy of the said phosphorescent emitter would be confined effectively and, as a result, the luminous efficiency would be enhanced. With the objective of improving this energy-confining effect, the triplet excitation energy is increased by modifying the structure of CBP in the non-patent document 1 and the luminous efficiency of bis[2-(4,6-difluorophenyl)pyridinato-N, C2′]iridium picolinate (hereinafter referred to as Flrpic) is improved by this means. Similarly, the luminous efficiency is enhanced by using 1,3-dicarbazolylbenzene (hereinafter referred to as mCP) as a host material in the non-patent document 2. However, these host materials are not satisfactory in practical use, particularly from the viewpoint of durability.
Moreover, the host material needs to have balanced electrical charges (hole and electron) injection/transport properties in order to enhance the luminous efficiency. The electron transport property is inferior to the hole transport property in the case of CBP and this disturbs the balance of electrical charges in the light-emitting layer and causes excess holes to flow out to the side of the cathode thereby reducing the probability of recombination of holes and electrons in the light-emitting layer and decreasing the luminous efficiency. Furthermore, in the case where an electron-transporting material like Alq3 whose energy level of the lowest triplet excited state is lower than that of Ir(ppy)3 is used, there may also arise the possibility that the luminous efficiency decreases due to transfer of the triplet excitation energy from the dopant to the electron-transporting material.
One of the means to prevent holes from flowing out to the electron-transporting layer is to provide a hole-blocking layer between the light-emitting layer and the electron-transporting layer. This hole-blocking layer accumulates holes efficiently in the light-emitting layer and contributes to improve the probability of recombination of holes and electrons in the light-emitting layer and enhance the luminous efficiency (the patent document 2). Hole-blocking materials in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato)aluminum (hereinafter referred to as BAlq). These materials can prevent holes from flowing out of the light-emitting layer to the electron-transporting layer; however, the lowest energy level of the excited triplet state of both of them is lower than that of a phosphorescent dopant such as Ir(ppy)3 and sufficient luminous efficiency cannot be obtained.
Moreover, BCP tends to crystallize even at room temperature and lacks reliability as a hole-blocking material and the life of the device is extremely short. Although BAlq is reported to have a Tg of approximately 100° C. and provide the device with relatively good life, its hole-blocking ability is not enough.
The aforementioned examples indicate that, in order for an organic EL device to perform at high luminous efficiency, a host material is required to have high triplet excitation energy and to be balanced in the electrical charges (hole and electron) injection/transport properties. Furthermore, the host material is hopefully a compound furnished with good electrochemical stability, high heat resistance, and excellent stability in the amorphous state. However, no compound capable of satisfying these properties on a practical level has been known at the present time.
The patent documents 3 and 4 disclose some compounds having a specified pyrimidine skeleton for use in organic EL devices. However, the patent document 3 merely discloses compounds which contain two or more pyrimidine-2,6-diyl groups having a conjugated substituent at the p-position and a method for preparing them and, although the document contains a description to the effect that they are useful as materials for organic EL devices, it does not verify their usefulness as such. On the other hand, the patent document 4 discloses compounds which contain a pyrimidine skeleton and can be used as a material for the light-emitting layer, but it discloses no compounds containing a bipyrimidine skeleton nor their use as a phosphorescent host material.